Our idea of leading-man material has changed. Today it's more about a rugged reality than flashing a megawatt matinee smile. Michael Fassbender's beautiful, evil turn in 12 Years A Slave, out this month, reminds us why the 36-year-old is the best actor for a generation. GQ goes mano-a-mano with the star his own director hails as the new Brando

Michael Fassbender has a story about the first and only play he ever put on in his hometown of Killarney, southwest Ireland, when he was just 18. The play was an adaptation of Quentin Tarantino's Reservoir Dogs. Not just any adaptation of Reservoir Dogs, however, but his adaptation of Reservoir Dogs. (This will become a significant detail.)

Fassbender - along with directing, producing and generally kicking the play into a muddled, chaotic, yet ultimately triumphant existence - played the jabbering Mr Pink ("He's got the best lines!") while his best friend from Fossa National School, Owen O'Shea, was cast as Mr Orange.

He's not vain. He's not precious. Michael's the boxer and I'm the trainer.

Steve McQueen

The half-Irish, half-German actor's parents, Adele and Josef, had endlessly worried that their son would end up behind the bar at the -family-run West End House, where his father was a chef. But before announcing his directorial debut, having left college the previous summer, he set up a theatre company. Just like that.

To be honest, this wasn't Fassbender's first foray into the world of artistic performance - if you ever meet the auburn-haired man with a grin like a great white, ask him about the Two Mikes, a band specialising in acoustic Metallica covers, and try not to laugh too hard - but it was his first serious attempt to do something he couldn't not do. (If that sounds melodramatic, that's kind of the point.) He called his company Peanut Productions. "Maybe because a peanut is small and can produce an awful lot of energy?" Big things; small beginnings. Coincidental prophecy- can be utterly comedic in hindsight.

The small big idea was inspired by a third-party Fassbender had met back in Killarney, a boy called Donny. Donny - a name Fassbender pronounces in that clench-jawed southern-Irish/German lilt as "D-own-y" - was a sharp kid who had gone off to the Gaiety School Of Acting, Ireland's pre-eminent drama centre. Donny was a little bit older than Fassbender. Maybe, at the time, a little bit cooler - if only through age and experience. It was, in fact, while attending one of Donny's amateur workshops that the future star of Shame, X-Men: Days Of Future Past (out in May) and this month's 12 Years A Slave first really caught the acting bug.

Fassbender even did a little part-time work for Donny - pantomime, some pub theatre here and there, although nothing too ambitious. After six months, however, the classes stopped, Fassbender struck out on his own, set up Peanut, and announced to his parents, to his older sister (now a brain surgeon), to Owen O'Shea, to Donny, to the whole town of Killarney - population 14,219 - that he would be staging a version of Tarantino's bloody masterpiece. On the day of the announcement Fassbender remembers feeling "like a god! For about an hour." Then, of course, the fear began to creep in. The self-doubt. The night sweats. "It was the terror," he admits. "The terror of everyone, the entire town, my parents, watching me fail."

In the past year Fassbender has spent just 20 days at his London home

Apart from those infamous jet-black suits (hired or borrowed or otherwise blagged by the actor) and the cop's severed ear (a lamb chop), everything else seemed to be a real problem. Even finding a venue, for example, was proving hard. The town hall was out. Some octogenarian spoilsport had deemed the material too crude, despite the actor insisting all profits were going to charity. (Years later Fassbender would tell Quentin Tarantino this story at his audition for the role he eventually won in Inglourious Basterds. According to Fassbender, Tarantino replied sort-of-jokingly-but-not-really, "So long as no one was making money out of my s$#!!")

Eventually, however, Fassbender caught a break. He got a stage. Well, a podium; the owner of Revels nightclub agreed to host. It was perfect. He knew he could use the strobe lights during the torture scenes and wash the stage with a red hue to heighten the menace. But despite the progress Fassbender couldn't shake the jitters. He was in over his head and he knew it. Even the other actors began to sense it.

Two weeks before curtain up, one of the older cast members staged a mutiny: "Hey Mike, maybe we should get Donny to have a look at some scenes..." Fassbender wobbled and panicked: "s$#!. Maybe he's right," he remembers thinking. "Maybe we need some -professional help here. Maybe we need f***ing Donny."

Fassbender went home that night raging, shredded with worry. He couldn't sleep. By morning he was pale - well, paler still - but he'd made up his mind. He walked into rehearsals, gathered his people and cleared his throat. "I told them, 'We're going to take the risk. We're going to do it ourselves.'" Reflecting on it today he adds, "I wasn't going to work for weeks on end and then allow Donny, anyone else, to waltz in and take the credit and save the show." The actor/director told his cast, "If it goes right, it goes right and if it goes to hell it goes to hell."

The older, belligerent actor wasn't buying his shaky bravado. He told Fassbender if he didn't pull the emergency cord and get Donny in fast he would quit then and there. "Quit then," spat Fassbender. "I'll recast you." From a dark corner of the rehearsal room one of the other actors mumbled something. It was O'Shea, "Everyone be cool!" Laughter broke the stand-off. The production ran for two nights. In Revels. With a lamb chop as a prop. First night saw 120 in attendance. Second night 140. A full house. Or at least a busy dance floor. No sign of Donny. "I learned all I know from that experience," chuckles the actor. "Everything."

Michael is an artist. There are actors and there are artists. Artists are people who fail, better," says Steve McQueen. If anyone helped reinforce and evolve the germ of an idea that began on a nightclub podium nearly two decades ago it is this director, who has cast Fassbender in all three of his acclaimed films, Hunger (2008), Shame (2011) and 12 Years A Slave. Tonight I'm at a hotel bar in Soho with Fassbender as the promotion for the film grinds, froths and fizzes towards the inevitable awards-season game of Guess Who?. Fassbender is in a talkative mood, though one doubts if he ever isn't. As McQueen will tell me on the phone a day or so later, "Michael just has a talent for being with people." Or as David Cronenberg put it on encountering an upbeat Fassbender day in, day out on the set of A Dangerous Method, "He's so perky it drives you crazy."

Women come with the territory. But he's not some cad. He's a gentleman.

Steve McQueen

McQueen's latest film is a step above even his own much-lauded cinematic projects. 12 Years A Slave - a shoo-in for well-deserved Oscar glory for those both in front of and behind the camera - is an adaptation of the memoir of the same name written by a man called Solomon Northup, first published in 1853. In the mid-19th century, Northup was lured to Washington, drugged and kidnapped from his placid life as an upstanding family man from Saratoga, New York - where he had been making a living as a concert violinist - bundled onto a disease-ridden slave ship and sold into forced labour amid the white-washed pillars of Louisiana's cotton plantations.

McQueen deals unflinchingly with the horrors endured by Northup and the other black slaves he meets on his shackled and bound journey, all the cruel horrors laid bare afresh in a film that demands that the audience witness the true savage inhumanity of slavery. Northup is played exceptionally by the British actor Chiwetel Ejiofor, while Fassbender inhabits the wretched, evangelical, psychotic mind of plantation owner Edwin Epps - a drunk, batterer and rapist who treats his human "property" worse than cattle.

In the book - Fassbender's first touch point when building the character for the screen - Northup describes Epps as "a large, portly, heavy-bodied man with light hair, high cheek bones and a Roman nose of extraordinary dimensions... When 'in his cups', Master Epps was a roistering, blustering, noisy fellow, whose chief delight was in dancing with his 'niggers', or lashing them about the yard with his long whip, just for the pleasure of hearing them screech and scream, as the great welts were plantedon their backs. When sober, he was silent, reserved and cunning, not beating us indiscriminately as in his drunken moments but sending the end of his rawhide to some tender spot of lagging slave, with a sly dexterity peculiar to himself."

Make no mistake, or let Northup's eloquent, balanced and often beautifully restrained prose sanctify the wretched, shameful reality: Edwin Epps was a monster. And Fassbender's -portrayal is of a man who embodies near--unwatchable evil. "He is a monster," agrees Fassbender, "but he too is a man. As hard as it is to believe, I had sympathy for him, despite his actions. I had this primal feeling that what we do to each other affects us, and that goes for both ends of the whip, if you like. I wanted to bring that part of Epps out to where the audience could recognise themselves, even if just for a second. So then it doesn't become so comfortable to keep his monstrosity at arm's length. Bring him closer and it's more effective. To force the audience into recognising the human being, the flesh and blood and brain behind the horror. Where does such abhorrent racism stem from? And what does it leave behind in a man like Epps?

"I remember I had to practise on the whip; I had to learn to crack it properly. I remember thinking about the knot it created in my shoulder; the knot it would create from doing such a physical task as whipping the skin off another human's back. That physical memory would stay in the body, my body, Epps' body. The body would retain the memory of what you have done to your fellow man or fellow woman. What would the psychological effect of this be? The retention of pain in your body memory? It's got to be stored somewhere. That's what I tried to do with Epps. There's a deep conflict in the man."

McQueen is emphatic in his admiration for his dear friend and star. "Michael is always looking to experiment and try something new or say something else; most actors just lean on what they know they can do well. Michael is a deep well. He doesn't have his shtick. He's willing to make himself look like an idiot to get where he wants to go. It's like painting the most beautiful landscape, then taking a cloth and wiping all that beauty away to start again. To be willing to throw all that good work out of the window. He's not vain. He's not precious. He's the boxer and I'm the trainer in that light. And every fight will be different. The worst thing to do is come to a fight rigid and prescriptive - something a lot of bad actors do. You need to be fluid."

When McQueen first cast the actor in 2008's Hunger, the story of the IRA hunger striker Bobby Sands, Fassbender - despite some success in the States with Spielberg's Band Of Brothers - was a rising star, yet to truly rise. Fassbender's first audition for McQueen went famously badly. "Although when he came back in," adds the director, "it was like seeing a different person." Today, does McQueen ever feel a pang of jealousy when Fassbender - who's potential has undoubtedly been unlocked by working with the director - goes off to work with the likes of Ridley Scott or Terrence Malick? "Yes, I will admit that I do! But you know he's going to be in demand so I have to begrudgingly accept that. He is the most influential actor of his generation, that is without doubt. He is Mickey Rourke or Gary Oldman in that way."

It's when you pull the chute that freaks me out. When you suddenly slow down... You can see all the details. Reality appears too close somehow.

Recently, Daniel Radcliffe has admitted to something he calls "the Michael Fassbender test". The Harry Potter star, when offered a role or a commercial deal, always checks the credibility of his actions by asking himself, "What would Michael Fassbender do?" McQueen agrees: "People want to be an actor because of Michael. Other actors now want to be attached to a particular film just because he is in it. That's how influential he is. He's unusual as he's a leading man who is also a character actor. I have to say the name Brando. Only Brando comes close as he's the only other actor who has that duality, both qualities delivered so well."

Talking to Fassbender as he sits at the bar and orders an Americano you feel you're catching a man mid-flight. Since his career hit fifth gear with Shame three years ago, his feet have hardly touched the floor. He has spent only 20 days in his flat in London's Hackney in the past 12 months. The only time off he's had was a short motorbike tour with his father and a trip to tour South America with an old friend, the pair making the decision one night after -indulging in too much red wine. "We were drinking a really great Malbec and we said, 'Hey, where's this from? Argentina? Let's go!'" Of course, as Fassbender's profile has risen so has fame's unremitting glare - the other side of being a modern movie star that generates an actor's heat in an industry that, on the whole, is about putting bums on seats rather than high art.

It was Shame, perhaps ironically, that transformed Fassbender into the sort of man your girlfriend or wife would swoon over. Her free pass. Her one one-night-stand. You have one of the two Kates (Upton or Moss) she has Fassbender. Even McQueen has noticed it: "Of course he's a sex symbol! I've seen the most stunning, beautiful, ladies approach Michael. I've seen it happen with my own two eyes. Blatant advances! He's an attractive man and it goes with the territory. It's maybe his openness once again. Men, women, whoever. But he's a gentleman. He's not some f***ing cad! He's a fun person to be around; guys want to hang out with him as much as girls want to go to bed with him. It doesn't matter - gay, straight or whatever - What a wonderful human being!"

Fassbender's beautiful co-stars couldn't agree more, it seems, as Natalie Portman - having shot a Terrence Malick project with him last year - confirms to me. "You lose your inhibitions around him that's for sure," she adds. "He's spontaneous and creative and one take is never the same, which is a breath of fresh air when you're having to do endless takes of the same scene. He frees you up. You don't worry about making a fool of yourself in front of him."

Did the Black Swan star and Academy Award winner see any of that "wild man" reputation come out on or off set? "I didn't, but I know he's a man who likes to enjoy himself!" And the romantic scenes? "Kissing? He relaxes you. It stops being a big deal. The pressure seems to be off when you're being close as he vanquishes any weird feelings. That sort of romantic scene can be weird - trust me, it's been weird for me in the past. But not with Michael..."

The 36-year-old actor does an excellent job of keeping out of the gossip pages, but he does have a wild-man reputation. I've seen the dipsomaniac within bubble to the surface once before, two years ago at a party to celebrate the opening of A Dangerous Method, held at 34 in Mayfair. He had to be carried out before the starters had even arrived, though today Fassbender insists he wasn't overly intoxicated. "I've had to be carried out of bars before. I was just larking around that night."

Shame had just come out and, aside from hailing his performance as the best of the year, some papers seem to be more interested in the size of his p****. Even George Clooney got up on stage at the Golden Globes in 2012, thanked Fassbender for taking over "the -frontal-nudity responsibility", pointed to a grinning Michael in the audience and added, "Hey Michael, honestly, if you can play golf like this, with your hands behind your back, go for it man!" I ask Fassbender if that sort of attention grates? "I mean, what am I going to do?" he laughs, shaking his head. "I couldn't stand up and be like, 'Stop talking about my p****!' I just had to take it on the chin. I still do. It is what it is. I was only worried that the subject and the power of the movie would be overshadowed by..." He stops himself, knowing full well that anything he says next will come out as a euphemism. He's been here before in interviews. "Whatever, man. I don't want to live in the past, I'm not like that. But if you're asking me did the d*** jokes get annoying at times, yeah, sure they did. I mean, just go and see the damn film, you know?"

I felt like a god. For about an hour. Then the terror of failure crept in.

Would all that schoolyard chat stop him doing full-frontal nudity again? "No! Of course I would do it again if the story needs it. When filming 12 Years A Slave, Steve wanted me to lose my pants for a particular scene and I was like, 'Steve come on, we've done Shame already!' Listen, it's not a big deal. I mean half the population has one between our legs and the other has something else. It's weird how there seems to be so much hype around one man's piece of anatomy! It's ludicrous really." Did his reputation ever affect his dating life? "It's weird I'm almost not aware of it. Maybe it's a defensive thing. I almost disassociate myself from 'that guy'. Like 'that guy' Clooney has made a d*** joke about on stage at a big Hollywood awards ceremony; like 'that guy' who is the movie star; like 'that guy' who supposedly has women falling at his feet. I don't see myself as a pin-up. I don't dwell on it. That sort of information isn't good for my own head."

Michael Fassbender has another story, One that perhaps explains where his head is at, about what he needs to do to wring life like a rag until it's dry and spent. Or it might be he's something of a hopeless adrenaline junkie. I'll let you decide.

Moving at an irresponsible speed is an important part of Fassbender's life. It's why he rides a Ducati - one that sounds like a space shuttle taking off. It's why he adores Formula One. It's why when I ask him what he would do if he could free up a little more time for himself he tells me emphatically, "Spend more time go-karting." It's either stop dead or be a blur. That's why he keeps the drinking in check nowadays. It's why he won't buy a TV for his flat: "I would end up slumped in front of it until 8am watching the God channel." It's why he can't ride a horse properly - he can sit stationary or gallop like a madman. There's no safe pace. No mediocrity.

"Have you ever been skydiving?" he asks me. I tell him I haven't and that I have no desire to. "Man, you've got to do it. I've done two jumps but I want to do more. Last time was in Hawaii, New Year's Eve. An incredible feeling. And that's the sort of thing that blows my mind about human beings. Someone somewhere first had that idea. To jump out of a plane and free fall." Doesn't he worry he could die? "No. Of course, you have to pull the parachute eventually and that's the bit that freaks me out. When you suddenly slow down and you can see how high you are. You can see the curvature of the earth.

It's weird how there's so much hype around one man's piece of anatomy.

"Applying the brakes suddenly makes you more aware of your own mortality. Before this, everything is just a rush of browns and greens as you scream towards the planet through space. Then the chute opens and you can see all the details. The cars. The trees. Houses. All of life is brought back into sharp focus. Suddenly reality appears a little too close somehow.

"Wouldn't it be terrible to be on your -deathbed and suddenly think, 'God, why didn't I do that? Was it because I was worried about failing? Or what someone else might think of me?' We are all going to die eventually, Jonathan. Take the jump. What have you got to lose?"